We have used quantitative ligand binding studies to characterize complex, heterogenous receptors for hormones and neurotransmitters in brain and other tissues. We have evaluated the hypothesis, that homocysteic acid (HCA) is a naturally occurring neurotransmitter for the N-methyl-D-Aspartate (NMDA) type of the glutamate receptor. The NMDA receptor is allosterically coupled to an ion channel, to which the drug MK-801 binds with high affinity and specificity. We examined the binding of [3H]-MK-801 to brain membranes which were thoroughly washed to remove endogenous amino acids and other small ligands. Repletion of L-glutamate, or L-homocysteic acid, resulted in parallel dose response curves for [3H]-MK-801 binding. This effect was potentiated by low concentrations of glycine, a known allosteric modulator of this system. More than 20 other amino acids were ineffective, demonstrating the specificity of L-HCA. A moderate degree of stereoselectivity was demonstrated. HCA appears to be selective for the NMDA receptor, and not effective at the kainic acid or quisqualate receptors. Together with data (e.g., electrophysiological) from other laboratories, these studies support the hypothesis that L-HCA may be an endogenous neurotransmitter at excitatory amino acid receptors. The effects of several monovalent and divalent cations on the binding of ligands to the "sigma" and phencyclidine (PCP) receptors have been examined in detail. These two types of receptors show differential sensitivity to ions. The sigma and PCP receptors of membranes from rat and guinea pig brain have been characterized using quantitative ligand binding studies with self-and cross displacement studies, dose response surfaces and extensive computer modelling. We demonstrate that sigma and PCP sites are distinct, and that the PCP sites can be resolved into high and low affinity forms for both rat and guinea pig, while the sigma receptor can be resolved into two forms in the guinea pig.